1. Field of the Invention
The present invention relates to a microprocessor system having an extended address space, and particularly to an address extending technique for a microprocessor system.
Microprocessors are easy to use, have a good cost performance, and therefore, are frequently employed in electronic controllers such as trunks or other equipment in an electronic exchange system. Such a microprocessor as mentioned above is, for example, an eight-bit microprocessor. They have, however, only a 64-kilobyte basic address space, which is not sufficient to store a large program for providing complicated functions. To solve this problem, an extension of the address space has been developed. Instead of the eight-bit microprocessor, a sixteen-bit microprocessor may be employed to provide a required address space. This, however, enlarges the circuitry and adversely affects the cost performance.
2. Description of the Related Arts
Cross references related to the present invention are: Japanese Patent Publication (Kokai) No. 62-25348 and Japanese Patent Publication (Kokai) No. 62-25348.
A conventional address space extending technique provides a basic address space with an extended address space that supplants a part of the basic address space. For example, the eight-bit microprocessor has a basic address space ranging from 0000 to FFFF, and for instance, a region of 8000 to 9FFF in the basic address space is supplanted by an extended address space ranging from 0000 to 8000. In this case, a basic memory is not provided for the region of 8000 to 9FFF of the basic address space, and instead a memory having a required capacity such as 8 kilobytes or smaller, is provided for the extended address space. This is possible when memory chips each having a memory capacity of 8 kilobytes are employed.
In recent years, however memory capacity has been increased greatly. Memory chips are no longer limited to a capacity of eight kilobytes (64 kilobits), but now comprise 32 kilobytes (256 kilobits) or larger. With such memories having a large capacity, e.g., a basic address space of, for example, 64 kilobytes may be filled by only two of 32-kilobyte RAM and ROM memory chips. Therefore, even when a part of one memory chip is to be supplanted by an extended address space, the whole space of the memory chip should be supplanted.
It may be possible to return to an address of the original 32-kilobyte memory chip after the extended address space has been used. This, however, requires a complex program including the steps to return to the original address space, resulting in a long processing time and a large memory space for the program.
Further, according to the conventional technique explained above, the part of the basic address space, 8000 to 9FFF, for example, supplanted by the extended address space may be a wasted space that is never used.